There are several methods for a transfer device used in an electrophotographic image forming apparatus. A widely known example among these methods is a transfer roller method using a conductive elastic member.
According to the transfer roller method, a transfer roller is disposed and rotated in press contact with a photosensitive member. A transfer bias of opposite polarity to that of toner on the photosensitive member is applied to the transfer roller to electrostatically attract the toner on a transfer medium.
As a method of controlling the transfer bias voltage applied to the transfer roller, Japanese Patent Laid-Open No. 4-335383 discloses a method of detecting the toner amount on a photosensitive member before transfer, and controlling the transfer bias in accordance with the detected toner amount such that if the toner amount is large, the transfer bias is decreased, and if the toner amount is small, the toner bias is increased.
However, this method does not detect the transfer efficiency during actual operation, and thus uses a table representing the relationship between the toner amount and the transfer bias that is prepared in advance by tests. Therefore, an optimal transfer bias cannot always be selected owing to environmental variations, changes in the resistance and thickness of the transfer medium in addition to the difference between the transfer roller used as a transfer means and the photosensitive drum. The method cannot satisfactorily prevent a transfer error and the like.
Japanese Patent Laid-Open No. 10-31375 discloses the following method. A toner image is transferred to a region at the end of a transfer roller where a transfer medium does not pass, and the density is detected and fed back to an image formation process control means to optimize image formation. Further, the reverse transfer bias is applied to the toner image adhering to the end of the transfer roller to reattach the toner image on a photosensitive drum, thereby cleaning the transfer roller.
However, the method disclosed in this reference suffers the following problems.
(1) Detecting toner adhering to the transfer roller is suitable for sensing variations in toner adhesion amount in developing operation under fixed transfer bias conditions. However, the toner adhesion amount after transfer is unknown, so the transfer efficiency cannot be detected.
Hence, an optimal transfer bias to be applied to the transfer roller cannot be obtained.
(2) The surface material (or the physical characteristics of the surface) is different between the image region and non-image region of the transfer roller. Optimal transfer bias conditions change depending on the surface material and surface characteristics. Thus, an optimal transfer bias actually necessary in the image region cannot be obtained.
When the transfer roller is used, the roller itself becomes dirty after repetitive printing operations. To solve this, a bias (cleaning bias) of opposite polarity to that of the transfer roller applied to the transfer roller in transferring an image to a transfer medium must be applied to the transfer roller when no printing is done, so as to reattach the toner on the photosensitive member. However, since the surface material of the transfer roller changes depending on the region, as described above, no optimal cleaning bias in the image region can be obtained.
As described above, it is difficult for the conventional methods to attain optimal transfer conditions for transferring a toner image from a photosensitive member to a transfer roller or transfer belt.